1. Field of the Invention
The present invention relates to a hetero-bipolar transistor, in particular, relates to an interconnection connected to electrodes thereof
2. Related Prior Art
A hetero-bipolar transistor (hereinafter denoted as HBT), in particular the HBT made of primarily group III and group V compound semiconductor material, provides a mesa structure for its major active region. A thickness of the mesa structure reaches sub-micron meters. Further, the semiconductor material used in the HBT has a crystal structure of the zinc blend type. When such thick semiconductor material having the zinc blend type structure is wet-etched, an anisotropic profile may occur in the etched surface. That is, a normal mesa surface, that makes an obtuse angle against the surface exposed by the etching, appears along [01-1] orientation, while a reverse mesa surface, which makes an acute angle against the surface exposed by the etching, appears along [011] orientation. Even in the surface along the [01-1] orientation, which appears the normal mesa surface, influenced by the reverse mesa surface along the [011] orientation, the width thereof is narrower in the root and the wider the closer to the top thereof.
A shape of the insulating film covering the mesa reflects the normal mesa surface and the reverse mesa surface, namely, a steep edge appears in direction along [011] orientation, and a gradual slope appears along [01-1] orientation. Therefore, an interconnection connected to the electrode formed within in the mesa is generally pulled out such that the interconnection intersects the surface along [01-1] orientation, in other words, runs along [011] orientation.
However, when the mesa region is so thick that the reverse mesa surface affects the shape of the normal mesa surface as previously mentioned, the width of the mesa region must be wide enough for the interconnection to the electrode within the mesa region to reliably put on the normal mesa surface. The wider mesa region results on the increase of the parasitic capacitance of the HBT, and degrades the high frequency performance thereof that is most expected in the HBT.
Therefore, one object of the present invention is to provide the HBT having a reliable interconnection without degrading the high frequency performance thereof